Mass spectrometry



E. E. HOSKINS MASS SPECTROMETRY Filed July 5, 1945 Jig-n Patented Apr. 24, 1945 2,374,205 MASS SPECTROMETRY Edmund E. Hoskins,

ring Corporation, Pasadena, Calif., a corporation of California Pasadena, Calif., assignor to Consolidated Enginee Application July 3, 1943Serial No. 493,415

(Cl. 'i3-18) 6 Claims.

This invention is concerned with mass spectrometers and provides improvements to the end that sensitive elements in the mass spectrometer, particularly the ion emission apparatus, the'ionization chamber, the analyzer and the equipment for evacuating these elements, are protected from damage due to power failures, gas leaks, and

other accidents.

A mass spectrometer is an apparatus for sorting ions, and may be employed for quantitative and qualitative analyses of mixtures, for the determination of isotope ratios. and for various other purposes. It usually comprises an ionization chamber and an analysis chamber plus anion collector. In the ionization chamber molecules of a substance being investigated are bombarded by ionizing particles such as electrons. These usually are emitted from an electrically heated lament, and shot as a beam against the molecules. The resulting ions are propelled by a potential impressed across o. plurality of electrodes into and through an analyzer chamber, where they are sorted by an electrical or magnetic field or both into a plurality of beams of ions having different mass-to-charge ratios, i. e. different specific masses. Each beam is composed of ions of the same specific mass and is collected separately, the current born by the ion beam being an index of the abundance of that kind of ion striking the collector.

The ionization chamber and the analyzer are operated in a highly evacuated state. The electron emitting element must also be operated in a vacuum. It `may be disposed in the ionization chamber proper or in 'an adjacent evacuated chamber. In either case, it is convenient to establish the required vacuum in the several parts of the apparatus by enclosing them in a common envelope of glass or the like which is evacuated by appropriate means, say a diffusion pump operated with oil or mercury followed by a high vacuum pump of conventional type.

The electron emitting element (filament) throws off metal when the pressure in the neighborhood of the element is too high with consequent excessive lament temperature. Under such circumstances, metal may be ldeposited on the interior surfaces of the apparatus. These deposits affect markedly the operation of the apparatus and should be avoided if possible.

The ion propelling electrodes are maintained during operation at high potential, of the order of i000 volts, so as to highly accelerate the ions and propel them through the iield in the analyzer at relatively high speed. If the pressure in the neighborhood of these electrodes is too high, undesired ionization of molecules, etc., may occur, and in aggravated instances there may be an actual arcing between the electrodes, with resultant compounding of gas with the metal surface to form undesirable deposits on the interior of the apparatus and with heavy current surges which may overload and damage various elements in the apparatus.

In accordance with my invention these and other difllculties are avoided by automatically interrupting thesupply of current'to the electron emitting element and to the ion propelling electrodes whenever the pressure in the neighborhood t these members, rises above a predetermined level. Protective relays are also utilized to prevent injury to the pumping apparatus.

'Ihese and other features of my invention will be understood more thoroughly in the light of the following detailed description, taken in conjunction with the accompanying single drawing which is a schematic diagram of automatic controls provided to safeguard the mass spectrometer in accordance with my invention.

Referring now to the drawing, it will be observed that a mass spectrometer provided with an analyzer tube Ill, an ionization chamber Il and an electron source in the form of a filament I2 are mounted within an envelope I3 adapted to be evacuated through a pipe line I4. The sample of gas to be analyzed is admitted into the ionization chamber through a conduit I5 connected to a s'ampling system I6 through a minute orice I Gas molecules entering the ionization chamber are bombarded by electrons of a beam I8 originating at the filament and propelled through the ionization chamber by a potential imposed upon an electrode or shield' 20. The molecules bombarded by the electrons are converted into ions. Vagrant electrons pass through the ionization chamber to an electron catcher 2 I. Ions formed in the ionization chamber are propelled to a slit Si at the exit of the chamber by a potential impressed between an electrode 22 which contains this slit and a pusher electrode 23 mounted in the ionization chamber. The ions are further propelled as a heterogeneous beam by a potential imposed between the slit S; and a second slit Sz in an electrode 2l in the head end of the analyzer tube. The two slits match each other and act as a collimator. From the second slit there issues into the analyzer tube a heterogeneous ion beam.

This beam is divided into a plurality of diverging homogeneous ion beams under the effects of a magnetic eld imposed in the analyzer tube. Thus the ions are separated according to their specific masses. If the magnetic field or the accelerating voltage or both are varied the separated ion beams may be caused to shift over the exit end lof the analyzer tube, which is provided with an exit slit S3 and a target 25 upon which ions are collected. The currents which resultl .from the collection of the several ion beams are amplified and recorded in conventional apparatus 26 to produce a mass spectrogram.

The ion accelerating voltages, which are applied between the pusher electrode 23 and the accelerating electrode 22 and between the accelerating electrodes 22-24, are supplied by a regulated high voltage D. C. powersupply 30, whichI infturn is supplied with power from a. normal lighting circuit or A. C. line 3|. The high voltage power supply feeds a beam sweeper 32 which in turn feeds the pusher electrode and the two accelerating electrodes. A relatively small potential is'maintained between the pusher electrode and iirst accelerating electrode 22 by means of small variable resistance 33. A relatively large potential is maintained between the two accelerating electrodes 22, 24 by means of a relatively large resistance 34. 'I'hus the resistances 33, 34 serve as a potential dividing network.

The second accelerating electrode, i. e. that at the head end of the analyzer tube, is `maintained at ground potential, as is one end of the large resistance 34.

The filament is supplied 'with curren't by a battery 35. A D. C. potential diierence is maintained between the filament and the surrounding shield 20 by a second battery 36. A further potential difference is maintained between the shield and ionization chamber wall, which includes the accelerating electrode 22, by means of a third battery 31. A fourth potential difference is maintained between the Wall of the ionization chamber and the electron catcher 2| by means of the battery 38.

The envelopeand consequently the filament, the ionization chamber and the analyzer tube which the envelope encloses are maintained at low pressure by the vacuum line I4, into which is connected a diffusion pump 50 of conventional type provided with a heater 5I, and preferably actuated by mercury. Following the diffusion or mercury pump is a second or fore vacuum pump 52 which exhausts to atmosphere.

To consider the automatic controls in the apparatus just described, it should be noted that a resistance 60 is provided in the lead to the first accelerating electrode 22. An increase of current following through this lead (above a predetermined maximum) actuates yan automatic control which cuts oii' the current feed to the regulated high voltage power supply 30. 'I'he two ends of the resistance 60 are connected, respectively, to the anode and cathode of a gas discharge tube 6I. An adjustable battery 32 is connected between the resistor 60 and the anode. A relay 63 is connected in the opposite side of the circuit between the resistor 60 and the cathode of the tube.

'I'he relay is provided with a set of contacts 64 which are normally closed. When the current flowing through the resistor 60 rises above the predetermined level. say 10 microamperes, the tube fires. 'I'he increase of current in the resistor 60 brings about a much greater increase in current through the tube, due to ionizationl therein. It is this phenomenon which causes the firing and the opening of the relay. When the relay is opened, the high voltage rpower supply V is interrupted on the primary side, thus cutting oif the potential applied to the accelerating electrodes in the spectrometer.

The iiring ofA the tube and consequent operation of the relay also opens the circuit connecting the filament I2, which is the electron source, and its associated battery 35, this being accomplished by a .pair of contacts 65 which are opened by the relay 63.

The purpose of the automatic cut-offs thus iar described is to protect the contents of the envelope against damage from dash-overs and particularly to protect the filament and the accelerating electrodes .and the interior of the ionization chamber, from contamination resulting from excessive current.

An ionization tyrpe vacuum gauge 1U with the usual associated circuits and apparatus, including a lament 1I is mounted in the conduit I4 between the em elope and diffusion pump and is provided to safeguard the apparatus in the event of the development of high pressure in the envelope. An increase in pressure in the envelope above a relatively low level tends to bring about excessive ionization therein with possible flashovers and contamination of clean surfaces within the apparatus, thereby destroying the calibration of the system. High pressure in the envelope would also destroy the filament. To prevent such occurrences, an increase in pressure above a predetermined level as indicated by the ionization vacuum gauge 10 is employed to interrupt the supply of current to the high voltage power supply and also to the filament I2 of the spectrometer. Thus the ionization gauge is provided with a relay 13 supplied with current by an associated source not shown. This relay is provided with a pair of contacts 14 which ar'e normally closed and are opened when the ionization gauge indicates an increase in pressure in the envelope above the predetermined level. The relay 13 also opens a second pair of contacts 15 which interrupts the circuit connecting the filament battery 35 and the filament I2 and a third pair of contacts 16 which interrupts the supply of current to the filament of the ionization gauge and prevents it from burning out. In this manner, the ionization gauge cuts off the supply of current to the filament and to the accelerating electrodesof the spectrometer and protects these elements of the apparatus and the interior surfaces of the ionization chamber from damage and contamination due to an increase in pressure, for example an increase caused by a leak in the envelope.

A Pirani gauge Buis connected to the conduit I4 between the diffusion pump and the fore-pump 52. -This Pirani gauge is of conventional construction and determines a change in pressure in the conduit between the diffusion pump and the viore-pump. If the pressure at this point rises above a predetermined level, a relay 8| connected to the Pirani gauge and supplied with current from a conventional source (not shown) opens a. pair of contacts 82, interrupting the primary side of the regulated high voltage power supply. At the same time, the relay opens a second pair of contacts 83 connected between the filament battery 35 and the filament I2. In this way, the supply of current to the lament and to the accelerating electrodes of the srectrometer is interrupted as described hereinbefore in connection with the ionization vacuum gauge. At the same time the Pirani gauge operates to interrupt the supply of current to the heating element of the difusion pump by means of a. relay 85 connected in series with the relay 8| and operating a pair of contacts 86.

The use of the Pirani gauge to automatically interrupt the heating of the diffusion pump saves this latter apparatus from damage in the event of a large leak in the system which supplies so much gas that the diffusion pump would become hopelessly overloaded and fouled.

The supply of current to the heating element 5| may be interrupted by a relay in the same manner as described hereinbefore in connection with the interruption of the supply of current to the primary of the high voltage power supply and to the filament.

In the operation of the apparatus described, the sample is admitted to the ionization chamber at a low but predetermined pressure. Preferably a sample of. gas to be vanalyzed is admitted into a sample bottle 90 of the sample system Ii through a valved inlet 9|, after the sample bottle has-fbeen evacuated through a valved line 92 by means of a diffusion pump 93 (preferably operated by oil) followed by a fore-pump 9| of conventional mechanical type. To prevent damage to the diffusion pump through overloading, for example when too large a quantity of gas is admitted into the sample bottle or when there is a leak in the system, the apparatus is provided with an automatic protective system similar to that employed between lthe diffusion pump 50 and the `forepump 52 for evacuating the envelope of the spectrometer. Thus a Pirani gauge |00 is connected to bombard molecules and convert them into ions..

means for supplying current to the electron emitting element, an analyzer, a plurality of elecare disposed, the combination which comprises a switch connected to said leads, and means responsive to an increase in the pressure in said space above a predetermined level for opening the switch.

3. In a mass spectrometer provided with an envelope, an electron emitting element mounted in the envelope for producing electrons to bombard molecules and convert them into ions, an

s analyzer, a plurality of electrodes for propelling ions thus formed through the analyzer, means for supplying current to the element to heat it and to the electrodes to impress a potential therebetween, and means for evacuating the envelope, the improvement which comprises means responsive to an increase in pressure in the envelope above a predetermined level for interrupting sielectrodes to impress a potential therebetween,

and means for evacuating the envelopeI the combination which comprises an ionization gauge having a filament disposed in the envelope, means for supplying current to the nlament, and means responsive to an increase in pressure in the envelope above a predetermined levell as indicated by the ionization gauge for Vinterrupting simul.

taneously the ow of current to the emitting element, the electrodes andthe filament.-

5. In a mass spectrometer provided with an envelope, an electron emitting element mounted in the envelope for producing electrons to bombard molecules and convert them into ions, an

trodes for propelling ions thus formed through the analyzer, means for flowing current to the electrodes to establish a potential therebetween, and means for evacuating the space within which the electron emitting element and the electrodes are disposed, the improvement which comprises means responsive to an increas in the pressure in said space above a predetermined level for interrupting the supply of current to the electron emitting element and to the electrodes.

2. In a mass spectrometer provided with an electron emitting element for producing electrons to bombard molecules and convert them into ions, an analyzer, a plurality of electrodes for propeiling ions thus formed through the analyzer, leads for owing current to the electrodes to impress a potential therebetween, and for ilowing current to the electron emitting element, and means for evacuating the space within which the electron emitting element and the elech'od analyzer, a plurality of electrodes for propelling ions thus formed through the analyzer, means for supplying current to the element to heat it and to the electrodes to impress a potential therebetween, and means for evacuating the envelope, the combination which comprises a conduit having a plurality oi vac um pumps connected in series to the envelo means connected in the conduit between the envelope and the grat pump or the series and responsive to an increase in pressure in the conduit above a predetermined level at the point o! connection for interrupting the iiow of current to the element and the electrades and. means connected in the conduit following the ilrst pump and responsive to an increase in pressure above a predetermined level in the conduit at its point of connection for interrupting the ilow o! current to the element and 

